The bug:

When an item is stuck in a full block (like stone and glass), it glitches trough the block(s) beneath it. It doesn't work with non-full blocks like stairs and grass path.

Steps to reproduce:

1. Make a floating platform.
2. Throw an item on top of the platform.
3. Place a full block at the place where the item sits.
4. If the item moves to the side, place another block.
5. The item will fall trough the platform.

If you do this on the ground, the item will fall trough the groud and, if there's no cave beneath it, into the void.

What is supposed to happen:

When an item is stuck in a block, it should get pushed upwards and end up on top of the block. (This is how item elevators work.) Instead, they get pushed downwards.